1,395 research outputs found
Group theory of Wannier functions providing the basis for a deeper understanding of magnetism and superconductivity
The paper presents the group theory of best localized and symmetry-adapted
Wannier functions in a crystal of any given space group G or magnetic group M.
Provided that the calculated band structure of the considered material is given
and that the symmetry of the Bloch functions at all the points of symmetry in
the Brillouin zone is known, the paper details whether or not the Bloch
functions of particular energy bands can be unitarily transformed into best
localized Wannier functions symmetry-adapted to the space group G, to the
magnetic group M, or to a subgroup of G or M. In this context, the paper
considers usual as well as spin-dependent Wannier functions, the latter
representing the most general definition of Wannier functions. The presented
group theory is a review of the theory published by one of the authors in
several former papers and is independent of any physical model of magnetism or
superconductivity. However, it is suggested to interpret the special symmetry
of the best localized Wannier functions in the framework of a nonadiabatic
extension of the Heisenberg model, the nonadiabatic Heisenberg model. On the
basis of the symmetry of the Wannier functions, this model of strongly
correlated localized electrons makes clear predictions whether or not the
system can possess superconducting or magnetic eigenstates
Decoherence and single electron charging in an electronic Mach-Zehnder interferometer
We investigate the temperature and voltage dependence of the quantum
interference in an electronic Mach-Zehnder interferometer using edge channels
in the integer quantum-Hall-regime. The amplitude of the interference fringes
is significantly smaller than expected from theory; nevertheless the functional
dependence of the visibility on temperature and bias voltage agrees very well
with theoretical predictions. Superimposed on the Aharonov-Bohm (AB)
oscillations, a conductance oscillation with six times smaller period is
observed. The latter depends only on gate voltage and not on the AB-phase, and
may be related to single electron charging.Comment: 4 pages, 6 figures, discussion of charging effect change
Interference and Interaction in Multiwall Carbon Nanotubes
We report equilibrium electric resistance R and tunneling spectroscopy dI/dV
measurements obtained on single multiwall nanotubes contacted by four metallic
Au fingers from above. At low temperature quantum interference phenomena
dominate the magnetoresistance. The phase-coherence and elastic-scattering
lengths are deduced. Because the latter is of order of the circumference of the
nanotubes, transport is quasi-ballistic. This result is supported by a dI/dV
spectrum which is in good agreement with the density-of-states (DOS) due to the
one-dimensional subbands expected for a perfect single-wall tube. As a function
of temperature T the resistance increases on decreasing T and saturates at
approx. 1-10 K for all measured nanotubes. R(T) cannot be related to the
energy-dependent DOS of graphene but is mainly caused by interaction and
interference effects. On a relatively small voltage scale of order 10 meV, a
pseudogap is observed in dI/dV which agrees with Luttinger-Liquid theories for
nanotubes. Because we have used quantum diffusion based on Fermi-Liquid as well
as Luttinger-Liquid theory in trying to understand our results, a large
fraction of this paper is devoted to a careful discussion of all our results.Comment: 14 pages (twocolumn), 8 figure
Liquid-induced damping of mechanical feedback effects in single electron tunneling through a suspended carbon nanotube
In single electron tunneling through clean, suspended carbon nanotube devices
at low temperature, distinct switching phenomena have regularly been observed.
These can be explained via strong interaction of single electron tunneling and
vibrational motion of the nanotube. We present measurements on a highly stable
nanotube device, subsequently recorded in the vacuum chamber of a dilution
refrigerator and immersed in the 3He/4He mixture of a second dilution
refrigerator. The switching phenomena are absent when the sample is kept in the
viscous liquid, additionally supporting the interpretation of dc-driven
vibration. Transport measurements in liquid helium can thus be used for finite
bias spectroscopy where otherwise the mechanical effects would dominate the
current.Comment: 4 pages, 3 figure
Negative frequency tuning of a carbon nanotube nano-electromechanical resonator
A suspended, doubly clamped single wall carbon nanotube is characterized as
driven nano-electromechanical resonator at cryogenic temperatures.
Electronically, the carbon nanotube displays small bandgap behaviour with
Coulomb blockade oscillations in electron conduction and transparent contacts
in hole conduction. We observe the driven mechanical resonance in dc-transport,
including multiple higher harmonic responses. The data shows a distinct
negative frequency tuning at finite applied gate voltage, enabling us to
electrostatically decrease the resonance frequency to 75% of its maximum value.
This is consistently explained via electrostatic softening of the mechanical
mode.Comment: 4 pages, 4 figures; submitted for the IWEPNM 2013 conference
proceeding
Counting Statistics and Dephasing Transition in an Electronic Mach-Zehnder Interferometer
It was recently suggested that a novel type of phase transition may occur in
the visibility of electronic Mach-Zehnder Interferometers. Here, we present
experimental evidence for the existence of this transition. The transition is
induced by strongly non-Gaussian noise that originates from the strong coupling
of a quantum point contact to the interferometer. We provide a transparent
physical picture of the effect, by exploiting a close analogy to the
neutrino-oscillations of particle physics. In addition, our experiment
constitutes a probe of the singularity of the elusive full counting statistics
of a quantum point contact.Comment: 7 pages, 4 figures (+Supplement 8 pages, 9 figures
Edge Channel Interference Controlled by Landau Level Filling
We study the visibility of Aharonov-Bohm interference in an electronic
Mach-Zehnder interferometer (MZI) in the integer quantum Hall regime. The
visibility is controlled by the filling factor and is observed only
between and 1.0, with an unexpected maximum near .
Three energy scales extracted from the temperature and voltage dependences of
the visibility change in a very similar way with the filling factor, indicating
that the different aspects of the interference depend sensitively on the local
structure of the compressible and incompressible strips forming the quantum
Hall edge channels.Comment: 5 pages, 5 figures, final version accepted for publication in Phys.
Rev.
Magnetic damping of a carbon nanotube NEMS resonator
A suspended, doubly clamped single wall carbon nanotube is characterized at
cryogenic temperatures. We observe specific switching effects in dc-current
spectroscopy of the embedded quantum dot. These have been identified previously
as nano-electromechanical self-excitation of the system, where positive
feedback from single electron tunneling drives mechanical motion. A magnetic
field suppresses this effect, by providing an additional damping mechanism.
This is modeled by eddy current damping, and confirmed by measuring the
resonance quality factor of the rf-driven nano-electromechanical resonator in
an increasing magnetic field.Comment: 8 pages, 3 figure
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